Tältning i Chamonix
Hej!
It's not necessary to wait until dark. If you're done with the day's program it's quite OK to put up the tent in the afternoon. Even more so in places with less traffic than Col du Midi. If one is aiming for "triple MontBlanc" route, though, it's quite wise to sleep at the cable car station Aigulle du Midi, in one of the upper corridors (they are warmer). If you opt for that, don't forget to take a big water bottle with you - saves a lot of gas. As for the things you want to leave in the village, the lockers at the campings cost about FRF 20 per bag/day, which is not that unbearable. But the risk that a car can be looted at one of the central parkings is also very small, I guess.
Good Luck
Alexey